I think my topic says it all, but since there is no header on the child views you get for View Schemes (the ones you get when Populate Tree is selected in the Edit View Scheme dialog), there is also no Thumbnail Size slider.
Now, I REALLY REALLY don't want any changes to the functionality of the View Schemes on the tree (I browse primarily using those "children" items), but it would be nice and more consistent if the header bar was added so that you could use the Thumbnail Slider from in there too.
Ideally in the future, your choice there would translate to a selection in Tiles View mode (meaning that if I open a Child Item in the tree (which should never show the tiles at first), then switch to the Tiles View mode it would keep the current tree item open in Tiles (whereas now they aren't connected). However, I realize this change would probably be fairly signifigant so it can certainly wait unti v12. However, if you gave us the View Header and simply disabled/greyed-out the "Options" and "Panes/Sub-Items" selections then I could actually use the wicked-cool Thumbnail size slider.